In recent years in a constitution of a surface light source device including an organic electroluminescent element (this may be appropriately referred to hereinbelow as “organic EL element”), lamination of a surface light-emitting body including an organic EL element and a light extraction film (a film having a concavo-convex structure on the surface thereof for enhancing light extraction efficiency) that is to be provided on the light-emitting surface of the surface light-emitting body is effected with an adhesive layer interposed therebetween. With such a surface light source device, light emitted from the organic EL element passes through the adhesive layer and the light extraction film, and is emitted to the outside of the device. In this surface light source device, a high refractive index of the adhesive layer is particularly advantageous for enhancing the light extraction efficiency. As a means for giving a high refractive index to the adhesive layer, there has been known a method of adding an inorganic oxide that is a high refractive index material to an adhesive composition that exhibits adhesive force (e.g., refer to Patent Literature 1).